1. Field of the Invention
The present invention relates to a family of transducers such as accelerometers and pressure transducers and more specifically to electromechanical sensors that utilize the principles of non-contact electron transfer mechanisms as the primary sensing element. The methods described herein are appropriate to the measurement of acceleration, pressure, force, torque and angular rate, and more generally to the measurement of any parameter where the input measured can be mechanically converted to a linear motion.
2. Description of the Related Art
As the modern technology of vehicles and moving machinery has advanced, it has become increasingly important to accurately measure the performance of such machinery. Feedback transducers are also employed in the automatic control of high performance machines. Furthermore, as the performance of such systems has increased, the demands on the sensors for accuracy and wide dynamic range have also increased.
Early sensors for measurement and control commonly employed mechanisms for converting the input parameter to a fairly large mechanical motion (e.g. spring mass systems for accelerometers and Bourdon tubes or diaphragms for pressure transducers) and a potentiometer or variable reluctance position sensor to detect the motion of the mass or Bourdon tube, thereby deriving an electrical signal proportional to the input measured. Later developments included the use of strain gages, both metallic and semiconductor, as the primary sensing mechanism, thus greatly extending the dynamic range of such systems. The introduction of closed loop or force balance sensors improved the accuracy of the sensors by orders of magnitude, but incur the penalty of decreased frequency response. Typical force balance sensors employ a sensitive motion detector (commonly a differential capacitor detector) and feedback of the amplified output to a force generating member (commonly a magnetic coil or solenoid) to balance the force created by the input measured. The principal advantage of closed loop sensors is their long term stability and inherent rejection of secondary inputs Since the restoring force acts to return the mechanism to near zero measured position, errors due to stability and linearity of the primary sensing mechanism are minimized.
Increasing sophistication of the apparatus employing sensing transducers, such as aircraft and space vehicles has placed heavy demands on the transducers for improved accuracy, precision and frequency response. In evaluating the relative "goodness" of various sensing mechanisms, it is convenient to examine a "figure of merit" which is the product of its sensitivity times the square of its frequency response. This figure of merit tends to be a constant for any given sensing system, thus it is possible to trade off sensitivity for frequency response and visa versa. To gain improvements in both sensitivity and frequency response it is necessary to innovate more efficient sensing mechanisms. In addition, the more stringent demands of the applications always tend toward to a "perfect transducer," that is, one that is perfectly accurate, has unlimited frequency response and that removes zero energy from the system being measured.
As the modern technology of vehicles and moving machinery has advanced, it has become increasingly important to accurately measure the forces experienced by systems and components. Furthermore, the performance of such systems has steadily increased, thereby placing greater demands on the accuracy and dynamic range of force measurement techniques. For example, the instruments developed to measure acceleration, generally designated as accelerometers, are a specific example of this class of instruments and have been designed in many different forms for various applications. Motion technology has developed the design of the accelerometer to meet the need for more reliable acceleration measurements, especially in dynamic ranges that were previously absent, unattainable or not of interest.
Among the practical applications of the accelerometer have been research into the dynamics of aircraft, spacecraft, and other higher performance moving vehicles to help understand the stresses and vibrations that can develop within the structures while undergoing acceleration. Other specifically designed accelerometers have been developed to detect earthquakes, control vibration test equipment, and instrument the test of products that are subject to sizable and often rapidly encountered accelerative forces. The accelerometer has also gained widespread use as an input source for advanced navigation and inertial guidance systems that utilize computers to determine guidance commands. In this application, the role of the accelerometer has changed from merely measuring acceleration for later analysis to actively forming a vital component in a system that controls the movement of a guided vehicle such as a missile, aircraft or spacecraft. In order to accommodate these and other demanding applications, the accelerometer must be rugged, accurate and have a large dynamic range while being capable of providing an output that can easily and accurately determine the acceleration of the test body or system.
While many accelerometers have been developed for a variety of applications, the most common designs utilize the same basic principles for measuring acceleration. An accelerometer is said to measure acceleration, but this statement is not quite true for methods commonly used to measure acceleration, since acceleration cannot easily be measured directly. Rather, an accelerometer usually measures the force exerted by restraints that are placed on a test mass that is subject to inertial forces when the accelerometer assembly is attached to an accelerating body. The acceleration can then be computed using the relationship between the restraint force and acceleration as defined by Newton's second law, namely F=Ma (force equals mass times acceleration). In most accelerometers, an electronic circuit is used to detect and measure the forces being exerted on the mass. Usually, the electric circuit is designed to measure the displacement of the test mass within its housing, since displacement of the mass restrained by an elastic member is proportional to the acceleration of the accelerometer system. For the most part, the design differences between most accelerometers are primarily the various ways in which the forces may be measured and converted to a proportional electrical signal.
Many accelerometer designs have been developed throughout the years due to the difficulty in adapting a particular design to the dynamic range and accuracy requirements of different applications. For example, some accelerometers may be used in extremely high-g ranges which would make highly sensitive accelerometers developed for other purposes, for instance, seismology unusable. When acceleration measurements are required for a system subject to many hundreds of times the force of gravity, or "g," piezoelectric accelerometers are often used. A piezoelectric accelerometer utilizes a test mass that is cemented to a piezoelectric crystal which is in turn cemented to a case or housing. The piezoelectric crystal supports the test mass and is strained in a predictable manner during acceleration. The stresses induced by the acceleration cause the crystal to generate a voltage which is proportional to acceleration.
Piezoelectric accelerometers are particularly noted for their small size and large dynamic range, which can be up to 5,000 g. For this reason, they are particularly useful in missile or spacecraft guidance systems that undergo very high accelerations.
While accelerometers are very useful for a wide variety of applications, there are a number of disadvantages associated with presently available instruments. For example, many of the electromechanical components used in accelerometers can be influenced by adverse ambient conditions, such as thermal fluctuations, which can alter the accuracy of the device and cause the accuracy to vary from 1 to 5%. Certain accelerometer designs can be made to have great sensitivity but because of this greater sensitivity, such accelerometers are susceptible to external, uncontrollable forces which may disable or reduce the accuracy of the device. In general, extremely accurate accelerometers are not readily used in applications which require the measurement of high-g accelerations. Therefore, the engineer or designer may have to use another, less sensitive accelerometer design in order to operate in a desired dynamic range.
Therefore, there is a need for a force sensing device which can operate at either a high or low acceleration environment and yet provide accurate measurements of forces experienced by a system or component. Such force measured devices would have wide application in modern systems, but would be particularly applicable to high performance accelerometers. Preferably, such an accelerometer must be rugged, durable, and small in size, especially if the accelerometer is to be used in applications involving guidance systems for missiles, spacecraft and other similar high-performance vehicles. The device must also preferably be relatively inexpensive to manufacture in order to reduce the total cost of any system of which it is a part. Additionally, the electronic circuitry needed to operate the device should be relatively simple to reduce the cost of manufacturing a commercial unit. Also, it would be desirable if the device could easily compensate for thermal expansion or contraction caused by temperature variations in the environment in order to eliminate errors in measurement associated with such thermal effects.